The present invention is related to digital communication systems and more particularly to systems and methods for ameliorating the effects of interference.
It is known to use adaptive spatial processing to exploit multiple antenna arrays to increase the communication quality of wireless systems. A weighting among antennas is chosen based on content of the signals received via multiple antenna elements. The spatial processor selects a weighting that optimizes reception of a desired signal while minimizing the deleterious effects of undesired signals.
OFDM (Orthogonal Frequency Division Multiplexing) is another highly useful communication technique. In OFDM, the available bandwidth is divided into subchannels that are orthogonal to one another in the frequency domain. A high data rate signal is effectively transmitted as a set of parallel low data rate signals, each one being carried over a separate subchannel. OFDM addresses a problem known as multipath caused by differences in delay time among different paths taken from a transmitter to a receiver. The effect of multipath is intersymbol interference created by energy associated with different symbols sharing a common arrival time. By creating multiple low data rate subchannels, OFDM lengthens the period occupied by a single symbol so that dispersive effects tend to be confined within a single symbol period, thereby reducing intersymbol interference.
Co-assigned U.S. patent application Ser. No. 09/234,629 discloses techniques for applying multiple antenna adaptive spatial processing to ameliorating interference in OFDM systems. In one of the techniques described there, an initial estimate is formed for the received data symbols independently for each antenna. This initial estimate is a so-called “hard decision” as to which data symbols was most likely transmitted. Based on this initial estimate, the noise and/or interference is also determined independently for each antenna. The noise and/or interference and its spatial profile is then statistically characterized over time and/or frequency based on the information received from all the antennas. The system then determines cost metric values suitable for input to a trellis decoding process based on the spatial statistical characterization and the raw frequency domain data symbol values as received via each antenna. The data symbol values from the different antennas are effectively combined so as to minimize the impact of interferers.
A problem arises in implementing this technique when there is heavy noise and interference. When a data symbol value becomes corrupted by noise and/or interference, a wrong hard decision results. Under strong noise and/or interference conditions, large numbers of wrong hard decisions degrade the spatial statistical characterization of noise and/or interference to the point that the system no longer optimally combines data symbol values from the antennas. Optimal interference cancellation no longer occurs and in effect, the system loses track of the spatial characteristics of the noise and/or interference and can no longer properly take them into account.
What is needed are systems and methods applicable to OFDM for effectively combining signals from multiple antennas to ameliorate the effects of even strong noise and/or interference.